ui-snapshot: improve extraction of revision from snapshot name

The modified get_ref_from_filename() supports the following snapshot
formats:
* $REV.$EXT
* $REPO[-_]*v?$REV.$EXT

This implies that the following urls will retrieve the expected revision:
* http://hjemli.net/git/cgit/snapshot/v0.8.1.tar.gz
* http://hjemli.net/git/cgit/snapshot/0.8.1.tar.gz
* http://hjemli.net/git/cgit/snapshot/cgit-0.8.1.tar.gz
* http://hjemli.net/git/cgit/snapshot/cgit-140012d7a8.tar.gz

Signed-off-by: Lars Hjemli <hjemli@gmail.com>